Expose `apps`?

`apps` are not exactly buildable, but their `program` can be evaluated (thus building the inputs):

```
❯ > nix eval .#apps.aarch64-darwin.cachix-push.program
/nix/store/nqi3ir7ffxj9sk9kz3q2rx7nv9vzpfnp-cachix-push/bin/cachix-push
```

